I first read about this practice in a horror comic, but I found out that it was a pretty well documented thing, known as Sokushinbutsu, where Buddhist priests of certain sects would starve themselves, ingest poison, then almost bury themselves alive (with breathing holes) with the hope of dying and then becoming a mummy. They are revered if they can become mummies. This site has a lot of interesting info, though the font color is the same color as the background so you have to highlight all the text.